1. Field of the Invention
The present invention relates to a chemical mechanical polishing apparatus used in manufacturing a semiconductor device. More particularly, the present invention relates to the polishing head of a chemical mechanical polishing apparatus, and to a method of washing contaminants off of the polishing head.
2. Description of the Related Art
Increasing the integration of semiconductor devices has required sequentially depositing multiple layers on a wafer. Accordingly, the semiconductor manufacturing process must include steps for planarizing each layer formed on the semiconductor wafer. Chemical mechanical polishing (CMP) is a typical process used for this purpose. In fact, CMP is well-suited for use in connection with large-diameter wafers because CMP produces excellent uniformity in planarizing wide areas in addition to narrow ones.
The CMP process makes use of mechanical friction and a chemical agent for finely polishing a wafer surface, such as that comprising tungsten or an oxide. In the mechanical aspect of such polishing, a wafer is placed on a rotating polishing pad and is rotated while a predetermined is load applied thereto, whereby the wafer surface is polished by the friction created between the polishing pad and the wafer surface. In the chemical aspect of such polishing, the wafer surface is polished by a chemical polishing agent, referred to as slurry, supplied between the polishing pad and the wafer.
A conventional CMP system will now be described with reference to FIGS. 1-7.
As shown in FIG. 1, the conventional CMP system includes a CMP apparatus 100 and a wafer transfer apparatus 170 adjacent to the CMP apparatus 100.
The CMP apparatus 100 includes a base 110, several polishing pads 120 installed on the base 110, a load-cup 130 for loading/unloading wafers 10, and a head rotation unit 140 having a plurality of polishing heads for holding the wafers 10 tight and rotating the same on the polishing pads 120. A protective cover 150 is installed on the upper portion of the base 110. The protective cover 150 surrounds and thereby protects the polishing pads 120, the load-cup 130 and the head rotation unit 140 . The protective cover 150 is also made of a transparent material which allows the polishing pad 120s, the load-cup 130 and the head rotation unit 140 to be viewed from outside the cover. An outlet 151 through which dust or other contaminants present inside the protective cover 150 are exhausted is provided on the protective cover 150 and is connected to an exhaust pump (not shown) through an exhaust pipe 152.
The wafer transfer apparatus 170 loads the wafers 10 into the CMP apparatus 100 and transfers the polished wafers 10 from one processing station to the next. The wafer transfer apparatus 170 includes a case 171 in which the wafers 10 are situated, a holder 174 for grabbing the wafers 10 from the case 171 and loading the same on the load-cup 130 of the CMP apparatus 100, an elevating arm for elevating the holder 174 and rotating the same, and a track 172 for supporting the elevating arm 173 and reciprocating the same in a predetermined direction.
The protective cover 150 also forms a wafer entrance 153 in one side thereof. The wafers 10 are loaded onto and are unloaded from the load-cup 130 by the wafer transfer apparatus 170 via the wafer entrance 153 in the cover 150.
Referring now to FIG. 2, a plurality of polishing pads, generally three polishing pads 120a, 120b and 120c, are installed on the base 110 so that a plurality of wafers can be processed at a time. The polishing pads 120a, 120b and 120c are fixed to a rotatable carousel (not shown). Pad conditioners 121a, 121b and 121c for adjusting the surface states of the polishing pads 120a, 120b and 120c, and slurry supplying arms 122a, 122b and 122c for supplying slurry to the surfaces of the polishing pads 120a, 120b and 120c, are disposed adjacent the polishing pads 120a, 120b and 120c. 
The head rotation unit 140 includes four polishing heads 141a, 141 b, 141c and 141d and four rotation shafts 142a, 142b, 142c and 142d. The polishing heads 141a, 141b, 141c and 141d hold wafers against the polishing pads so that a predetermined pressure is exerted thereon while the wafers are being polished. The rotation shafts 142a, 142b, 142c and 142d for rotating the polishing heads 141a, 141b, 141c and 141d, respectively, are mounted on a frame 143 of the head rotation unit 140. A driving mechanism for rotating the rotation shafts 142a, 142b, 142c and 142d is provided within the frame 143 of the head rotation unit 140. The head rotation unit 140 is supported by a rotary bearing 144 so as to be rotatable about the longitudinal axis of the bearing 144.
Furthermore, the load-cup 130 includes a circular pedestal 131 which supports the wafers. As will be described later, the polishing heads 141a, 141b, 141c and 141d are washed at and by the load-cup 130.
The operation of the CMP apparatus having the structure described above will now be described with reference to FIGS. 2 and 3. First, the wafer 10 transferred to the load-cup 130 by the wafer transfer apparatus 170 is placed on the surface of the pedestal 131 of the load-cup 130. There, the wafer 10 is vacuum-chucked on the surface of the pedestal 131 so as to be fixed in place. Then, the wafer 10 is lifted using the pedestal 131 and is thereby transferred to a polishing head 141 positioned above the pedestal 131. The wafer is then vacuum-chucked to the polishing head 141, and the head rotation unit 140 is rotated to transfer the wafer to a position above the polishing pad 120a adjacent to the load-cup 130 . Then, the polishing head 141 is lowered so that the wafer 10 is pressed tightly against the polishing pad 120a. The wafer is polished in this state while slurry is supplied between the wafer and the polishing pad 120a. In this case, the polishing pad 120a and the wafer 10 rotate in the same direction, typically counter-clockwise. Each wafer 10 is transferred sequentially among the three polishing pads 120a, 120b and 120c and then to the load-cup 130 where the wafer is placed on the pedestal 131. Thereafter, the wafer transfer apparatus 170 transfers the wafer 10 placed on the pedestal 131 to the outside of the CMP apparatus 100.
Once the wafer 10 has been unloaded, the polishing head 141 is lowered towards the load-cup 130. In such a state, deionized water is sprayed to wash the bottom of the polishing head 141 and the pedestal 131. When the washing is completed, the polishing head 141 and the pedestal 131 are raised and a new wafer is transferred by the wafer transfer apparatus 170 to the pedestal 131.
Next, as shown in FIGS. 4 and 5, a first nozzle 135 and a second nozzle 136 for spraying deionized water are provided in a washing basin 132 of the load-cup 130. The first nozzle 135 is oriented so as to spray deionized water toward the top surface of the pedestal 131, and the second nozzle 136 is oriented so as to spray deionized water toward a membrane 1411 installed at the bottom of the polishing head 141. The membrane 1411 allows a vacuum to be exerted on the wafer held by the polishing head. Three sets of the first and second nozzles 135 and 136 are spaced from each other at equal angular intervals about the pedestal 131. Three wafer aligners 133 for guiding a wafer into position are installed within the washing basin 132 of the load-cup 130. The wafer aligners 133 are also spaced form one another at equal angular intervals abut the circumference of the pedestal 131.
The washing basin 132 is supported by a cylindrical support housing 134. A flexible hose 1352 for supplying deionized water to the first and second nozzles 135 and 136 is installed within the support housing 134. A washing fluid channel 1351 connects the flexible hose 1352 to the first and second nozzles 135 and 136 within the washing basin 132.
A plurality of spray orifices 1311 in the pedestal 131 spray deionized water upwards to wash the membrane 1411. A lateral passageway 1312 in the pedestal 131 connects the spray orifices 1311. The lateral passageway 1312 is connected to a vertical passageway 1313 formed inside a tubular pedestal column 138 which supports the pedestal 131.
As described above, the load-cup 130 is responsible for washing the membrane 1411 on the bottom of the polishing head 141, for washing the pedestal 131, and for supporting wafers during the loading and unloading thereof onto and from the CMP apparatus 100. The washing step is very important in the CMP process. Contaminants such as slurry debris or polished silicon particles are unavoidably produced during the CMP process, and some of the contaminants may remain on the surface of the membrane 1411 and/or the pedestal 131. The contaminants remaining on the surface of the membrane 1411 and/or the pedestal 131 can generate micro-scratches on the wafer surface if the contaminants are transferred thereto when the wafer is loaded in the course of polishing. The micro-scratches may cause defects such as gate oxide leakage or gate line bridging in the semiconductor devices, which lowers the yield and reliability of the semiconductor devices. Thus, any contaminants remaining on the membrane 1411 and/or the pedestal 131 must be removed by washing the same using deionized water.
However, such contaminants cannot be completely removed by the washing operation performed by the conventional CMP apparatus. This washing operation will now be described with reference to FIGS. 6 and 7.
The polishing head 141 of the CMP apparatus holds a wafer thereto under a predetermined amount of pressure and rotates the wafer in such a state. More specifically, the wafer is held by a vacuum to the polishing head 141 while it is rotated. To this end, a vacuum line 1419 is provided within the polishing head 141, and a membrane support plate 1414 having a plurality of holes 1415 communicating with the vacuum line 1419 is installed at the bottom of the polishing head 141. A membrane pad 1416 is closely fixed on the bottom of the membrane support plate 1414. A membrane pad 1416 is fixed close to the bottom of the membrane support plate 1414. The bottom of the membrane pad 1416 and the outer surface of the membrane support plate 1414 are surrounded by the membrane 1411, which is made of a flexible material which comes into direct contact with wafers. The membrane 1411 is fixed to the membrane support plate 1414 by a membrane clamp 1417. A retainer ring 1412 for preventing wafers from deviating outwards during polishing is disposed at the lower outer edge of the polishing head 141, that is, at the perimeter of the membrane 1411. Four purge holes 1413 are provided at the outer circumference of the retainer ring 1412 at equal angular intervals. While a wafer is adhered to the membrane 1411, air can enter/leave a small space 1418, formed between the membrane support plate 1414 and the retainer ring 1412, via the purge holes 1413.
In the polishing head 141 having the structure described above, a narrow gap having a width (D) of about 0.254 mm is present between the membrane 1411 and the retainer ring 1412 so that the membrane 1411 can be elevated with respect to the retainer ring 1412 when a load is applied to a wafer. However, the slurry or contaminants produced during polishing are induced into the space 418 through the gap having the width D. The slurry or contaminants induced into the space 1418 can not be and are not removed by the washing means of the load cup 130. Also, as shown in FIG. 7, although four purge holes 1413 are provided in the retainer ring 1412, the diameters thereof are at most 2 mm. Thus, the contaminants induced into the space 1418 cannot be exhausted through the small purge holes 1413. Thus, the contaminants accumulate over time and solidify as moisture evaporates therefrom.
The solidified slurry or contaminants drop onto the surface of a polishing pad due to vertical movement of the membrane 1411 or slight vibration of the polishing pad during polishing. The size of the contaminants which drop onto the surface of the polishing pad exceed several micrometers, whereby micro-scratches or even macro-scratches can be formed in the surface of a wafer.
Nonetheless, the conventional CMP apparatus is provided with means for exhausting contaminants such as slurry debris or polished silicon particles while preventing the contaminants from being diffused into a clean room and in an attempt to prevent the wafers from being contaminated.
Referring again to FIG. 1, the exhaust means includes the outlet 151 provided on the protective cover 150, an exhaust pump (not shown) and the exhaust pipe 152 connecting the outlet 151 with the exhaust pump. The slurry debris or polished silicon particles typically present inside the CMP apparatus are larger and heavier than particles in the air and are generally produced on the polishing pad 120 at a location remote from the outlet 151. Accordingly, the contaminants present inside the CMP apparatus cannot be effectively exhausted through the conventional exhaust outlet 151 and therefore remain inside the apparatus. These residual contaminants may scratch the wafer surface.
As described above, contaminants such as polished silicon particles or slurry debris are not completely removed in the conventional CMP apparatus. Thus, the surfaces polished by the CMP apparatus can become scratched, thereby lowering the yield and reliability of semiconductor devices produced from the wafers polished by the CMP apparatus .
It is an object of the present invention to solve such problems of the prior art by providing a chemical mechanical polishing (CMP) apparatus for and a method of effectively removing contaminants which would otherwise scratch the wafer surface.
More specifically, it is a first object of the present invention to provide a CMP apparatus having washing means which can effectively remove contaminants such as slurry debris adhered to the inside of a polishing head of the apparatus.
It is another object of the present invention to provide a CMP apparatus having exhaust means which can effectively exhaust contaminants such as slurry debris adhered to the inside of the apparatus.
It is still another object of the present invention to provide a method of washing away contaminants such as slurry debris adhered to the inside of a polishing head of a CMP apparatus.
To achieve the first object, the present invention provides a chemical mechanical polishing (CMP) apparatus for planarizing the surface of a semiconductor wafer, the CMP apparatus including first, second and third spray nozzles. The first nozzles are disposed within a load-cup where the loading and unloading of the wafers takes place and spray deionized water toward the top surface of a pedestal on which the wafers are supported so as to wash the pedestal. The second nozzles are disposed within the load-cup and spray deionized water toward a membrane provided at the bottom of a polishing head of the apparatus so as to wash the membrane. The third nozzles are also disposed within the load-cup, and spray deionized water through purge holes in a retainer ring of the polishing head, toward a space formed between the outer surface of the membrane and the inner surface of the retainer ring, whereby contaminants induced into the space are washed away.
Preferably, at least three of the third nozzles are disposed along the inner surface of the load-cup at equal intervals. In one embodiment, an annular deionized water supply line extends along the inner surface of the load-cup and the third nozzles are installed in the deionized water supply line at predetermined intervals. The number of third nozzles is preferably the same as that of the purge holes formed in the retainer ring.
To achieve the second object, the CMP apparatus further includes exhaust means comprising an outlet, an exhaust pump and an exhaust pipe connecting the outlet and the exhaust pump, for exhausting contaminants through a protective cover, the outlet being located at the bottom of a side of the protective cover.
Preferably, four of such outlets are provided, one at each of four side surfaces of the protective cover. Each outlet defines a rectangular opening in the protective cover.
To achieve the third object, the present invention provides a method of washing contaminants induced into a polishing head of a CMP apparatus, wherein the contaminants are washed by spraying deionized water into a space in which the contaminants are present through purge holes formed in a retainer ring located at the lower peripheral portion of the polishing head.
Preferably, the deionized water is sprayed by nozzles installed within a load-cup where the loading and unloading of the wafers takes place. Also, the deionized water is preferably sprayed while rotating the polishing head.